Mac Pro 3.1 power issues


I have moved my mac pro 3.1 motherboard from a badly damaged case to another case. The power light is permanently on and the machine fails to power up. i have taken the cmos battery out and tried a different cmos battery but still no change. I can now get the machine to power up by plugging the power cable in as long as no cmos battery is fitted. The power light is still on permanently and the machine seems to be working ok when the os is loaded. I am currently windows 10 to test the board and prime95 seems to be ok up to now. I have swopped the front usb board and tried a different psu but still having power issues.
What else can i try?
thanks for the reply.
Not to start with as i wanted to test the old psu from the dead mac pro 3.1. but after issues i have gone back to the working psu. still the same problem. power light always on and no post with cmos battery in. I have tried 2 cmos batteries and tested them with multimeter and they both seem fine. one was pulled from machine with no date issues.I am currently running apples hardware test to see if it comes up with anything.
You might work through the troubleshooting steps in Apple's service manual. If you don't have a copy, you can search for the file name - macpro_early2008.pdf
thanks for the link. i will follow the troubleshooting guide later. At the moment i am running the AHT software and it does not report any faults apart from a rear exhaust fan failure ( it is not plugged in at the moment ). I am currently running prime 95 on a win 10 OS install carrying out further tests.